Update README as AsciiDoc and with a demo GIF

Author: suy

README

@@ -0,0 +1,22 @@
+== vim-context-commentstring
+
+This is a very simple plugin to automatically set the 'commentstring' and
+'comments' Vim options in file types which contain more than one syntax (like
+JavaScript nested inside HTML, or Lua nested in VimL).
+
+It uses the names of the syntax highlighting groups to know where the cursor is,
+and just set the value of those Vim settings to one appropriate for that part of
+the file.
+
+This will make Vim properly use the features, which by default are just fixed
+values which are set by file type. This allows to use a plugin like
+https://github.com/tpope/vim-commentary[Tim Pope's vim-commentary] to toggle the
+comments, or the native Vim functionality of wrapping text when you are typing
+a lines of comments.
+
+image::demo1.gif[Demo]
+
+See the documentation in the doc/context-commentstring.txt file, or browse it
+online from the hosted source code in:
+
+https://raw.github.com/suy/vim-context-commentstring/master/doc/context-commentstring.txt